1. Field of the Invention
The invention relates to a node and more specifically but not exclusively to a node within a mobile communications device.
2. Description of the Related Art
User equipment have become application rich personal devices capable of more than voice communication. It is for example difficult to purchase user equipment which does not feature a digital camera connected to internal digital image processing elements, polyphonic audio synthesizing equipment. It is also common for user equipment to be connected or incorporate advanced features such as satellite navigation and audio and video recording and playback.
In order that the components of the user equipment can communicate with each other user equipment is equipped with a communications link or network designed in such a way that internal systems can communicate with each other to generate this functionality, and that external systems can also be connected to the user equipment, to enable the user equipment to be upgradeable.
A known example of such a communication network is the serial interface known as D-Phy (proposed by the mobile industry processor interface alliance (MIPI)). The D-Phy serial interface supports as many as four lanes operating at rates up to 1 Gbit per second per lane and uses low-voltage, source-synchronous, scalable-signalling technology.
Operating on these physical networks are protocol stacks. The protocol stacks define how data is transmitted across the physical network. For example the MIPI unified protocol (UniPRO) defines standards for transmitting data packet over the D-Phy network.
The current MIPI UniPRO standards and the protocols used by other proprietary user equipment networks suffer from the problem of data integrity—in other words relying that the data transmitted by the data originator (or first end node) of the network has been received and is being processed by a data final destination (or second end node). In these networks the nodes have a limited capacity for receiving and processing data. Thus when the final destination node reaches the capacity the node can not physically receive or process the any further data transmitted through the network.
In these situations the typical network protocol allows implements either a stop or discard process.
The stop process instructs the final destination node to stop accepting traffic from the network. This results in “head of line blocking” where the data being transmitted is queued in the network nodes between the two end nodes. This queued data causes the network nodes to effectively block the node from passing any further data until the next node accepts the current data packet. This blocking therefore propagates from the final destination node to the originator node resulting in partial or full blocking of the network.
The discard process instructs the final destination node to drop a packet. This allows the network to operate efficiently and without blockage, but results in a loss of end-to-end (E2E) reliability as even if the data is reaches the final destination node the data can not be guaranteed to have received and processed the data.
Point-to-point (P2P) data flow control is known. In such systems the data transmitted between two immediate nodes can be acknowledged to confirm its receipt. These P2P systems can be divided into two groups.
The first group employs flow control without synchronisation information by using flow control tokens. One example is the Spacewire system. These systems have further problems in that they are required to employ complex mechanisms for recovering loss of the flow control tokens. Furthermore it is difficult to estimate the data overhead of transmitting control tokens which prevent such systems from producing accurate quality of service provisions in bandwidth limited networks.
The second group of P2P systems operate flow control with synchronisation information. These typically apply techniques synchronising the flow control information at both ends with every flow control signalling packet and therefore are complex and require a greater signalling overhead than the token based group. An example of this synchronised flow control signalling can be found the P2P flow control mechanism featured within the current MIPI UniPRO specifications.
Using the P2P flow control mechanisms for both point-to-point and end-to-end reliability produces additional complexity with regards to signalling overhead and complexity of nodes within the network whilst producing a system which is not optimal for end-to-end flow control.